The present invention relates to a device for measuring minute elongations by means of a light wave conductor-sensor and to an arrangement of such a device or structural parts whose elongations are to be monitored.
Light wave conductors (LWC), which term is equivalent to "sheathed optical fibers" have been already formed into and installed in various sensors. One of the most abundant groups of such sensors includes LWC-sensors for mechanical forces (such as tension-, stress-, or compression-, bending- and torsional forces), which make themselves apparent through changes in length (such as elongation, compressing strain or buckling, bending and twisting stress). The indication of such changes in length is the corresponding change in the light damping.
A prior art LWC-sensor for tension forces and its application for monitoring a bridge structure of prestressed concrete is described in German application DE-C2-33 05 234. In this known embodiment a light wave conductor is embedded in a tension proof wire of a fiber reinforced resin structure so that the wire can be monitored as to its tension, breakage or bending. For this purpose the LWC is enveloped in layer of synthetic material of a non-homogeneous structure; the LWC, the intermediate layer and the wire are mechanically firmly connected one to each other over the entire length thereof and the LWC is provided on both ends thereof with connectors for a light passage testing apparatus. The sensivity of this LWC tension sensor is further increased by winding at least one coil of a metal wire (steel wire) or of a glass fiber around the LWC to provide the non-homogeneous intermediate layer (DE-A1-35 26 966).
From the multitude of the different known LWC sensors only the following types will be mentioned; there are LWC sensors for small radioactive radiation doses whereby the light damping is an indication of the dose (DE-U1-82 18 823); and LWC-sensors for the electric current intensity whereby the Faraday effect is utilized as an indicator. For example, known is an optoelectronic current converter for measuring current in a high voltage cable which operates with Faraday effect in a monomode-LWC (Z. etz. volume 106 1985, 1160). Due to the Faraday effect the polarization plane or linear polarized light propagating (in the monomode-LWC) in the direction of magnetic field lines is rotated by the action of the applied magnetic field. To determine from the Faraday effect the current values, the LWC-sensor is coupled at one end thereof to a polarizer and a light source and at the other end thereof to an analyzer and an evaluation electronic.